The invention relates to a low-pressure mercury discharge lamp having a color rendering index Ra,8 between 80 and 90, a color point on or near the Planckian curve and comprising a gastight lamp vessel which allows passage of visible light, contains Hg and one or more inert gases and is provided with a luminescent screen including
a first luminescent substance which emits predominantly between 520 nm and 565 nm, and PA1 a second luminescent substance which emits predominantly between 590 nm and 630 nm.
The color rendering index Ra,8, as well as the color rendering index R9 mentioned later in this description, is defined as in "A user-oriented description of color rendition of light sources" by J. T. C. van Kemenade, CIE 1995 Congress. A low-pressure mercury discharge lamp, hereinafter also referred to as lamp, as mentioned in the opening paragraph is disclosed in U.S. Pat. No. 4,176,294 and in Dutch patent 164,697. During operation of the lamp, the first and the second luminescent substance emit, respectively, green and red light while the Hg present in the discharge causes a quantity of blue light to be directly generated. As a result of the green, red and blue contribution to the luminous flux of the lamp, the color of the light generated by the lamp is white. Since the quantity of blue light directly generated by Hg is too small to generate white light having a high color temperature, the luminescent screen generally comprises, in addition to the first and the second luminescent substance, a further luminescent substance which emits blue light during operation of the lamp.
The color of visible radiation is characterized by the color co-ordinates (x,y) which determine the color point in the color triangle (see Publication CIE, No. 15 (E-1.3.1.), 1971). A low-pressure mercury discharge lamp for general lighting purposes should emit light which can be regarded as being "white". In the color triangle, white radiation is found at color points situated on the Planckian curve. This curve, also referred to as line of the full radiators, and hereinafter referred to as the P curve, includes the color points of the radiation emitted by a completely black body at different temperatures (the so-called color temperature). A specific color temperature is not only assigned to a specific point on the P curve, but also to radiation whose color co-ordinates are situated on a line intersecting the P curve at this point (see said Publication CIE, No. 15). If this radiation has a color point near the P curve, this radiation is also considered to be white light with said specific color temperature. In this description and in the claims, "a color point near the P curve" is to be taken to mean that the distance from the color point to the point on the P curve having the same color temperature is maximally 20 MPCD. MPCD (minimum perceptible color difference) is the unit of chrominance, see the publication by J. J. Rennilson in Optical Spectra, October 1980, page 63. In addition to requirements to be met by the luminescent layer in connection with properties relating to the color temperature of the light emitted by the lamp and the color rendition, the luminescent layer must also comprise substances which, upon excitation by ultraviolet radiation generated in a low-pressure mercury discharge, yield a high luminous flux and which maintain this high luminous flux during the service life of the lamp.
The known lamp has a good color rendition (Ra,8&gt;80) and a relatively high luminous efficacy (up to 90 lm/W and higher) and hence can very suitably be used for general lighting purposes. Tests have revealed that while lamps having a much better color rendition (Ra,8&gt;90) are more appreciated by test personnel, the luminous efficacy is relatively small.